When you create a new class you'll be prompted to assign a teacher and (if the teacher doesn't have a default payrate), you'll be asked to choose one for that class.
Side note: here's how to update a teacher's default pay rate.
If you don't see a pay rate in the dropdown menu that works for this class, you can create a new pay rate right from the class creation page.
The video below covers how the basic pay rate settings work.